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ABSTRACT: Late Quaternary deposition, Mahakam River delta Indonesia: stratigraphic and sedimentological elements of a mixed siliclastic deltaic and carbonate shelf system modulated by eustatic cycles

Roberts, Harry1, Johan Sydow2, Barun Sen Gupta1, and John H. Wrenn1
(1) Louisiana State University, Baton Rouge, LA 
(2) BP Amoco, Houston, TX

The first comprehensive high-resolution seismic and side-scan sonar data sets collected on the Mahakam delta shelf produced a new understanding of the offshore parts of this tropical depositional system. The data base for this three-year study included over 3000 line km of offshore high resolution acoustic data and onshore/offshore bottom samples (380) and vibracores/piston cores (97). These data identify mixed siliclastic and carbonate depositional systems, the dominance of which is modulated by late Pleistocene-to-Holocene eustatic sea level cycles, giving the shelf a unique sedimentary architecture.

The modern highstand delta is prograding across a shelf dominated by numerous individual and fused bioherms built primarily by the calcareous green alga Halimeda. A strong N-S current from the Makassar Strait sweeps across the shelf and restricts deposition of prodelta sediments to ~20 km off the central delta front as compared to over 50 km to the south. The remainder of the shelf is dominated by carbonates, resulting in mounded biohermal topography established on a regional transgressive surface developed as sea level rose above the shelf edge following the latest Pleistocene glacial maximum. This surface was established on top of falling stage Pleistocene deltaic deposits, which burn and preserve bioherms in siliciclastic sediment. Deltaic and shelf sediments are clearly distinguished by two distinct foraminiferal biofacies. Ammonia beccarii, Nonionella sp., and Elphidium advenum represent the deltaic deposits while Amphistegina papillosa, Operculina ammonoides, and Calcarina spengleri reflect the carbonate shelf. Similarly, diversity of spores, pollen and organic matter clearly distinguish Holocene deltaic sediments from marine organic facies of the middle and outer shelf. Seismic data suggest these stratigraphic and sedimentologic relationships in an alternately dominant siliciclastic-carbonate depositional system are replicated throughout the late Pleistocene as a product of eustacy.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia